1. Field of the Invention
The present invention relates to a power control device, and particularly relates to a power control device that can switch to a bypass relay when the power is in full output, thus the temperature rise of the elements such as motor soft starter, solid state relay, thermistor, etc can be controlled. In addition, its simple structure not only reduces size and cost, but also simplifies the whole design.
2. Description of Related Art
A power control device is often applied in electric equipment to adjust the power volume, instead of only connecting the electric equipment to the power supply through an electromagnetic relay. For example, when a motor is activated, full voltage will be input, leading to an excessive current and a pull-in torque which may be harmful to the motor. So, a power control device is applied to control the voltage change at start and stop moments, whereby the voltage will not change sharply at those two moments and the motor is protected.
The power control device generally employs SCRs (silicon controlled rectifier) to realize the buffering process, however, if the current passes through the SCR during the whole operation, the SCR may become overheated, therefore a cooling fin is provided to dissipate excessive heat, but the cooling fin will increase the size of the equipment. Another way to reduce the heat rate is to provide an electromagnetic relay, and when the motor is in a stable operation, the current is switched to the electromagnetic relay, but that will further increase the size and the design is more complex.
To have both the advantages of the electromagnetic relay and the power control function, it is more preferable to combine the two elements to a single control device. With reference to FIG. 5, a motor soft starter has a relay built in. The starter has two main circuit boards (50), (52), a relay connecting board (53) with three relays (51) mounted thereon, and a triggering circuit (58) with three circuit boards (54) mounted thereon corresponding to the three relays (51). Each circuit board (54) has two SCRs (55) mounted thereon, and a heat sink (60) is fitted opposite each SCR (55). Every circuit board (54) has a first conducting strip (57) and a second conducting strip (59) respectively provided at opposite ends, and the first conducting strip (57) is at the current input end, and the second conducting strip (59) is at the current output end. The two conducting strips (57), (59) are connected to the relay connecting board (53). The input and output of the three relays (51) are respectively connected to the input and output of the SCR. When the motor is in stable operation, the current is made to pass through the relay, but the relays (51) take up most of the space, leaving little space for the cooling fin (60).
Therefore, the invention provides a power control device to mitigate or obviate the aforementioned problems.